1. Field of the Invention
The present invention relates to microwave type concentration measuring apparatus that measures the concentrations of fluid bodies to be measured which contain various suspended solids or soluble substances, such as sewage sludge, pulp or building materials.
2. Description of the Related Art
Hitherto, as equipment for measuring the concentrations of suspended solids and the like, ultrasonic type concentration meters, which find the concentration by measuring the attenuation factor of ultrasonic waves, and optical type concentration meters, which find the concentration using light by measuring the attenuation factor of transmitted light or the increase of scattered light, have been widely used.
However, with ultrasonic type concentration meters, if, for example, bubbles are intermixed in the fluid, these have a great effect and there are large measurement errors. Also, with optical type concentration meters, if dirt or the like adheres to the optical windows used for projecting the light or receiving the light, this, too, has a great effect and there are large measurement errors.
Therefore, recently, as a concentration meter which does not undergo these influences of bubbles in the fluid and dirt on optical windows, a microwave type concentration measuring apparatus has been conceived and is about to be put into practical use.
FIG. 1 shows a schematic block diagram of this type of microwave type concentration measuring apparatus. This is composed as follows. The microwave transmitting antenna 2 and the microwave receiving antenna 3 are located opposite each other and are attached on the outside wall of pipe 1 which contains the flowing fluid to be measured. The microwaves pass through the fluid to be measured which is flowing in pipe 1. Microwaves from microwave generator 4 are inputted to microwave transmitting antenna 2 via power splitter 5. The microwaves transmitted from microwave transmitting antenna 2 are diffused in the fluid under measurement inside pipe 1, received by microwave receiving antenna 3 and inputted to phase difference measuring circuit 6. At the same time, microwaves from microwave generator 4 are directly inputted to phase difference measuring circuit 6 via power splitter 5. Then, phase lag .theta.2 of the microwaves coming diffused by the fluid under measurement in pipe 1 behind the microwaves directly transmitted from microwave generator 4 via power splitter 5 is found by this phase difference measuring circuit 6. Next, this phase lag .theta.2 is compared with phase lag .theta.1, which is that measured beforehand by filling pipe 1 with a reference fluid (for example, tap water, which can be regarded as zero concentration) and measuring the microwaves coming diffused by the reference fluid in the same state as the fluid under measurement. The design is to find the concentration X of the fluid under measurement from the resulting phase difference .DELTA..theta.=(.theta.2-.theta.1) using a calibration curve such as that shown in FIG. 2. This concentration X can be found based on calibration curves of X=a.DELTA..theta.+b corresponding to each type of fluid under measurement. Here, a is the gradient of the calibration curve, and b is the intercept. Normally, b=0.
When using this type of microwave type concentration measuring apparatus, the phase difference of the microwaves is measured, not the attenuation factor. Also, there is no need for any windows through which the microwaves are projected and received to be transparent. Therefore, the measurement apparatus is not easily affected by bobbles or dirt, making it possible to continuously measure the concentration of the fluid under analysis.
However, in a case such as a fluid under measurement containing a number of substances, if the constituent composition of one of these substances should vary, the measurement sensitivity would change due to the effect of that. Thus there were times when errors which could not be ignored occurred in concentration measurement results for fluids under measurement.
Now, for example, consider the case of mixture concentration measurement of a fluid under measurement in a state in which a compound, in which substances A, B and C are mixed in constant proportions, is suspended in water. If it is taken that the constituent (molecules and elements) composition of substances A and B do not vary but the constituent composition of substance C does vary, the variation of the constituent composition of that substance C will produce a great effect on the measurement sensitivity of the mixture of substances A, B and C as a whole. For example, when simple carbon is contained in substance C and the percentage content of that carbon varies, that variation will appear as a variation of the measurement sensitivity of the mixture as a whole.
Also, if the measurement sensitivity of the mixture as a whole varies, errors will occur in the results of concentration measurement of the fluid under measurement. The reason for that can be considered as the following.
Here, for the measurement sensitivity of a microwave type concentration measuring apparatus that uses the phase difference measurement method, by how much the phase difference varies for a 1% concentration (weight %) variation when expressed by an equation becomes: EQU Measurement sensitivity=(Phase difference .DELTA..theta.)/(Concentration X)=(1/a)
and is expressed by the inverse of gradient a of the calibration curve. Here, intercept b of the calibration curve=0.
By this means, if the measurement sensitivities of substances A, B and C as respective simple bodies are taken as (1/a1), (1/a2) and (1/a3), measurement sensitivity (1/a0) of the mixture as a whole is determined by (1/a1), (1/a2) and (1/a3) and the mixture proportions of substances A, B and C. Therefore, supposing that measurement sensitivity (1/a3) varies due to variation of the constituent composition of substance C, measurement sensitivity (1/a0) of the mixture as a whole will also vary. In this case, if correction is not carried out by taking the constituent composition of substance C at the time of a certain state as a reference, concentration measurement errors will occur.